During the course of manufacturing an integrated circuit, the need frequently arises to fill an opening in the form of a hole or trench (generally in the surface of a dielectric layer) with material such as tungsten or copper. A particular example of this is the well-known damascene process in which conducting lines are formed that are flush with the surface rather than lying on top of it.
We illustrate, in FIG. 1, a particular problem which arises when a hole or trench needs to be filled with copper. This problem becomes increasingly more acute as the diameter or width of the opening gets to be less than about 0.3 microns. Before the opening can be filled using a high-volume technique such as electroplating, it is necessary to lay down a seed layer of copper. In FIG. 1 we show a cross-section of a portion of dielectric (or some other material) layer 11 in which opening 15 has been formed. Layer 12 is a layer of field oxide that is normally present although not directly relevant to the practice on the present invention.
The problem mentioned above arises with the deposition of seed layer 13 (usually, but not necessarily) of copper. The most widely used processes for depositing the seed layer are vacuum evaporation and sputtering. (known collectively as PVD or physical vapor deposition). Because of shadowing effects, there is a tendency for more material to build up near the mouth of the opening than lower down, giving the deposited seed layer the profile shown in FIG. 1. In particular, there can be substantial overhang of the seed layer, as pointed to by arrows 14, near the mouth of the opening.
When layer 13 is later built up, typically by electroplating, so as to fully fill opening 15, material at the edges of the overhang come together before the hole can be fully filled, resulting in the trapping of a void within the copper plug (sometimes called the key hole effect).
An obvious approach to dealing with this problem is to reduce the amount of overhang to the point that void trapping does not occur. In the prior art this is done either by limiting the amount of copper deposited in the first place or by etching the seed layer back using conventional chemical means, such as wet or dry etching.
Simply reducing the thickness of the seed layer, while reducing the possibility of void formation, introduces a new problem which is illustrated in FIG. 2. Shown there is seed layer 23, having reduced thickness relative to seed layer 13 of FIG. 1. However, if the overhang 24 is sufficiently reduced, bare spots such as 25 begin to appear on the side walls of the opening. The presence of such bare spots then has disastrous consequences for the subsequent hole filling procedure since multiple voids and poorly adherent areas get formed. The present invention shows how a seed layer having little or no overhang, while at the same time having sufficient thickness to fully cover the side walls, may be formed.
Effect on contact resistance between wiring layers: We note here that, because of the problems outlined above, it was necessary to limit trench depths so as to keep their aspect ratio (as seen in cross-section) to less than about 6:1, otherwise there was a danger of voids forming when they were filled with metal. This in turn meant that via holes extending downwards from the trench bottom to the next wiring level (i.e. dual damascene structures) had to be correspondingly deeper (i.e. their aspect ratio would typically be at least 6:1). As a result, the series resistance of the via (i.e. wiring level to wiring level contact resistance), when filled with metal, would be larger than desired. Typically, a contact resistance less than 1 ohm for a via size of 0.2 microns could occur. In FIG. 6 we show a typical dual damascene structure of the prior art in which the depth of trench 61 is T1 and the depth of via 62 is V1. It follows that any increase in the ratio T1/V1 will reduce the wire-to-wire contact resistance.
A routine search of the prior art was conducted but no references were found that teach the solution described by the present invention. Several of these references were, however, of interest. For sample, Crank (U.S. Pat. No. 5,316,974) limits the seed layer to the bottom of the trench so that the filler plug grows (by electroplating) from the bottom up and not from the vertical sides, thereby avoiding void formation.
Zhao et al. (U.S. Pat. No. 5,674,787) form the seed layer by exposing the conductor at the bottom of the hole and then dipping it in a solution that deposits a thin copper layer by displacement. A plug is then grown on this seed layer using an electroless process.
Venkatraman (U.S. Pat. No. 5,677,244) dopes aluminum with copper by first laying down an agglomerated copper film, then depositing the aluminum copper then heating so as to diffuse the copper islands throughout the aluminum.
Venkatraman et al. (U.S. Pat. No. 5,814,557) describe a process for filling the trench/hole of a damascene structure by depositing two different conductors one after the other.